According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collection a number of samples. A set of samples of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
Magnetic resonance angiography (MRA) has been shown to allow accessing the status of arteries and blood vessels of patients. In the present invention, contrast-enhanced MRA (CE-MRA) is considered, in which MR images are acquired during the arterial first pass of a paramagnetic contrast agent after intravenous injection. However, the intravenous injection itself is not part of the invention.
In conventional contrast-enhanced MR angiography background suppression is obtained by repetitive RF pulses. In this way, most background signal is effectively suppressed except for the fat signal. In order to also suppress the fat signal, which might obscure the vasculature of interest, generally two acquisitions are made: one before injection of the contrast agent—the so called ‘mask’ image—, and one during presence of the contrast agent. The mask image is then subtracted from the contrast image to eliminate the fat signal.
Although conventional CE-MRA using subtractions as described above has been used for many years, there are several disadvantages to the technique. First at all, this technique requires 2 acquisitions, namely one before and one after contrast agent injection, which increases total scan time. Further, in case motion occurs between the mask and the contrast scans, a subtraction of the resulting images might not be possible due to misalignment of some image features. This problem may even become more severe in areas where a breath hold is required in this case subtraction is difficult, as two breath holds are never identical.
An alternative to subtracting the fat signal from the images is to perform fat suppression during the acquisition of the MRA images. For fat suppression various types of approaches are known. For example chemical shift selective pre-saturation (SPIR, SPAIR) or chemical shift selective excitation strategies can be applied to either suppress or not excite fat signal However, these pulses are too time consuming to build into a CE-MRA scan, as the available scan time is very limited in first pass imaging.
An alternative approach to eliminate the fat signal from the images would be to use water-fat separation by the Dixon method. However, the conventional Dixon method is not well suited for contrast-enhanced MR angiography, since it requires the acquisition of two echoes at echo times, at which water and fat signals are in- and opposed-phase. This leads to long echo times (TEs) and thus to long repetition times (TRs), rendering the timing of the acquisition with arrival of the contrast agent in the region of interest impossible. The conventional Dixon method can therefore generally not be used for CE-MRA, as the technique is too slow.
From the forgoing it is readily appreciated that there is a need for an improved MR imaging method. It is consequently an object of the invention to enable MR imaging in a fast manner by providing CE-MRA images with fat suppression for first pass imaging. Further, from the forgoing it is readily appreciated that there is a need for an improved MR imaging system and an improved computer program product adapted to carry out the method according to the invention.